中性氢区（ H区）是一种由中性氢原子组成的星际云。这些区域并不明亮，但是会辐射出21公分（1,420MHz）谱线。这条谱线的发生机率很低，所以须要有很大量的氢原子存在才能看见这条谱线。当有游离区域在前方时，H区会与扩张的游离气体（像是电离氢区）碰撞，而只要被游离的区域达到H区的10（也就是一万个中有一个），发出的光就会比21公分波更为明亮。

使用电波望远镜描绘H的辐射是用来测量螺旋星系漩涡臂的一种技术，也可以用来描绘星系之间的交互作用造成的重力扰动。当两个星系发生碰撞时，如同绳索般被抽离出来的物质，让天文学家可以测量星系是如何移动的。

An HI region (read H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. These regions do not emit detectable visible light (except in spectral lines from elements other than hydrogen) but are observed by the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where HI regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would. The degree of ionization in an HI region is very small at around 10 (i.e. one particle in 10,000). At typical interstellar pressures in galaxies like the Milky Way, HI regions are most stable at temperatures of either below 100 K or above several thousand K; gas between these temperatures heats or cools very quickly to reach one of the stable temperature regimes. Within one of these phases, the gas is usually considered isothermal, except near an expanding H II region. Near an expanding H II region is a dense HI region, separated from the undisturbed HI region by a shock front and from the H II region by an ionization front.